1. Technical Field
The present invention relates a switching power source device that has first and second switching elements subjected to on/off-driving alternately and that has a function of adjusting a dead time width that regulates the turn-on timing of the first and second switching elements.
2. Related Art
As schematically illustrated in FIG. 4, for example, a resonance-type switching power source device includes a first switching element Q1 connected in series to a primary coil T1 of an insulation transformer T and a second switching element Q2 connected in series to the first switching element Q1 and connected in parallel to the primary coil T1 of the insulation transformer T. The first switching element Q1 switches an input voltage Vi applied through an input capacitor Ci and electric power is accumulated in a leakage inductor of the insulation transformer T. Electric power accumulated in the inductor by switching the second switching element Q2 is transferred from secondary coils S1 and S2 of the insulation transformer T to an output capacitor Co using resonance of the inductor, and thereby an output voltage Vo is obtained (for example, see Patent Documents 1 and 2).
The first and second switching elements Q1 and Q2 are formed from power transistors such as a high-withstand voltage MOSFET or IGBT, for example. Moreover, in FIG. 4, a resonance capacitor Cr is a resonance capacitor connected in series to the primary coil T1 of the insulation transformer T. A voltage generated between the secondary coils S1 and S2 of the insulation transformer T is rectified by rectification diodes Da and Db and is transferred to the output capacitor Co. Moreover, the switching elements Q1 and Q2 include body diodes D1 and D2.
The switching operation of the first and second switching elements Q1 and Q2 is controlled basically as follows. An error detector 1 detects the output voltage Vo obtained in the output capacitor Co, and the output voltage Vo is fed back to an oscillator 2 that regulates a switching frequency of the switching elements Q1 and Q2. The oscillator 2 performs a role of outputting an OFF trigger signal Off_trg in response to a feedback signal (an error signal between a preset output voltage and an output power Vo) from the error detector 1 at a timing that occurs a predetermined period that regulates the ON width of the switching elements Q1 and Q2, later than the input timing of an ON trigger signal On_trg to be described later.
The control circuit 3 alternately generates driving signals Lo and Ho for turning on the first and second switching elements Q1 and Q2, respectively, according to the ON trigger signal On_trg and the OFF trigger signal Off_trg and drives a driver circuit 4 using these driving signals Lo and Ho to turn on the first and second switching elements Q1 and Q2.
The ON trigger signal On_trg used for generating the driving signals Lo and Ho is generated by a dead time adjustment circuit 5 that inputs the OFF trigger signal Off_trg. This dead time adjustment circuit 5 performs a role of adjusting the timing at which one of the switching elements Q1 and Q2 is turned on after the other of the switching elements Q1 and Q2 is turned off. Specifically, the dead time adjustment circuit 5 generates the ON trigger signal On_trg at a timing that occurs a predetermined delay time (the dead time Tdead) later than the input timing of the OFT trigger signal Off_trg. The dead time Tdead is set based on the time required for a voltage VS applied to the switching elements Q1 and Q2 to reach zero (0 V) due to a current flowing through the body diodes D1 and D2 in response to the turn-off of the switching elements Q1 and Q2.
However, the time (turn-off rise/fall time) required for the voltage VS to reach zero (0 V) in response to the turn-off of the switching elements Q1 and Q2 changes depending on a parasitic capacitance of the switching elements Q1 and Q2 and a resonance current flowing through the inductor. Specifically, the turn-off rise/fall time of the switching elements Q1 and Q2 increases in a light load state in which the resonance current is small. Conversely, the turn-off rise/fall time of the switching elements Q1 and Q2 decreases in a heavy load state where the resonance current is large. That is, the timing at which the voltage VS reaches zero (0 V) changes depending on a load state.
Therefore, conventionally, the bottom (peak) of the voltage VS generated due to the resonance current of the inductor in response to the turn-off of the switching element Q1 (Q2), for example, is detected from a coil voltage Vp2 generated in the auxiliary coil T2 of the insulation transformer T, and the dead time Tdead is adjusted according to the detection result. Specifically, a dV/dt detector 6 differentiates the coil voltage Vp2 generated in the auxiliary coil T2 of the insulation transformer T and detects the timing at which the polarity of the differential value is reversed as the timing of the bottom (peak) of the voltage generated due to the resonance current. The dead time adjustment circuit 5 automatically adjusts the dead time Tdead according to the timings P2L and P2H detected by the dV/dt detector 6.
The dead time Tdead is adjusted to be between a smallest dead time for preventing simultaneous turnon of the first and second switching elements Q1 and Q2 in a heavy load state and a largest dead time for forcibly restarting the switching operation in a light load state or when it is not possible to detect the reversal of the polarity of the coil voltage Vp2.
Patent Document 1: Japanese Patent Publication No. 2005-51918
Patent Document 2: Japanese Patent Publication No. 2010-4596